Lecture 1. Lecture 1: The Different Modalities

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Gilbert Barnett
5 years ago
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1 Lecture 1 Lecture 1: The Different Modalities In this Lecture Understanding the difference between the different modalities available Learn when to chose the appropriate modality Trust me, during the next few lectures, you won t be learning about reading radiographs. You won t learn how to diagnose anything for that matter. So what are we doing here? Well, before you can learn to read a radiograph (or any other test for that matter) you must first learn when to order that particular test. For example, If your sister has a hamster with a swollen belly and you had to pick one test to figure out what is going on, which test would you chose? Radiographs? CT? Ultrasound? MRI? If you pick the wrong one you don t get any information. You waste your sisters money and there is a family crisis. If you pick the right one, you get an answer and everyone is happy. (In this case, an ultrasound of the abdomen is probably the best answer.) The following section is an introduction to what we do in the radiology department at the AUCVM. When I went through vet school, I would generally sleep through the introductory lecture in every course. They never took test questions from those lectures and everything was usually repeated somewhere in the following lectures. Not this time. Expect to be asked in the not too distant future a question like the one above about the squirrel. And without further adieu. the different modalities available at Auburn are

2 Diagnostic Radiography Also known as x-rays or radiographs. Currently, this is the bread and butter of veterinary radiologic diagnostics. You will all use x-rays. That goes for team bovine, team equine, and of course team dog and cat! Nearly every practice has access to an x- ray machine. Get used to reading radiographs as soon as possible. Basis: To create a radiograph, x-rays are passed through the patient and an image is recorded on a piece of radiographic film. That is all I am going to say for now. You have all seen them and we will go in painful depth on this matter in subsequent lectures. Best For: Radiographs are the single best screening test we have in the department of radiology. Radiographs are inexpensive (xxx for a 2 projection thorax at Auburn) and generally the animal doesn t need to be anesthetized or sedated. Moreover, the animal feels no pain. (For that matter none of the imaging modalities we will discuss in this class cause any pain for the patient.) With radiographs, we can look at the whole abdomen, the whole thorax, or a whole body for that matter; we can look at a lot of places at once. Radiographs give us the most information about tissues that have a high intrinsic radiographic contrast. These include Bones surrounded by air or soft tissue Lungs The outside border of the heart as it is surrounded by air Radiographs, may not be as valuable at evaluating structures in the abdomen. This is because the tissues of the abdomen all kind of look the same, or have a similar opacity, on a radiograph. Don t get me wrong, radiographs of the abdomen are quite valuable, however, they are a little tougher to read than say a radiograph of a femur. Disadvantages: There are, however, disadvantages: Exposure to ionizing radiation: X-rays damage tissue. The less radiation you are exposed to, the better. We can only justify the use of x-rays if the benefit of the information gleaned outweighs the cost of the exposure to radiation Superimposition of structures: Radiographs create a 2D image of a 3D structure. Therefore, superimposition of overlying structures is a problem. Always take two projections. Terminology: We describe radiographs in terms of opacity.

3 Ultrasound Also known as echo or sonogram, ultrasound is becoming more and more popular in veterinary medicine. The cost of machines is coming down and many employers expect you to be experienced in ultrasound when you graduate. Basis: A piezoelectric crystal is used to generate sound in the range of millions of hertz per second (we cant hear it and neither can your dog). This sound passes into tissue and bounces off different tissues in varying degrees. The ultrasound probe then listens for the echo produced by the sound bouncing off the tissue and creates and image. The image we see is a real time image meaning that we can see the heart beat, the bladder contract and we can even see blood flow with something called Doppler ultrasound. Ultrasound is safe. The sound used in diagnostic ultrasound (pretty much) cannot hurt the patient and there is no exposure to ionizing radiation. Additionally, we don t have to sedate animals. Best For: Ultrasound is used to look at soft tissues such as the abdomen, heart, muscle, tendons, and even the brain and joints. It is used extensively in large animal reproduction (uterus, ovaries etc.) We cannot use ultrasound to look at bones or air filled structures such as the lungs. Sound cannot propagate effectively in air or bone so we cannot create an image of these structures. We can, however, look at the surface of bone and lung. Disadvantages: Like I said before, we cannot look at bone or gas filled structures. That said, we do plenty of ultrasound examinations at Auburn. However, it s gonna cost ya. The going price is xxx for an abdominal exam. The information gleaned can be invaluable so, when applied in the right circumstances, it is worth it. The reason it costs so much the equipment is expensive. Our machine is $225,000! The ones you will see in practice are $20,000 to $80,000. Also, it takes about an hour to complete an ultrasound exam. Terminology: We describe ultrasound images in terms of their echogenicity.

4 Nuclear Medicine Also known as nuclear scintigraphy or just nuc med, this modality is used less frequently in veterinary medicine than the previous two modalities. Some of you going into equine medicine will have scintigraphy in your practice. The others will have to refer patients to a veterinary college or other large specialty practice. Therefore, you really need to know what the indications are and when to refer them in. Basis: Radioactive agents (known as radiopharmaceuticals) are given to a patient and the amount of radiation that is emitted from a particular part of the patient is recorded with a camera called a scintillation detector or gamma camera. Some definitions are in order: Scintillation (the root of scintigraphy): When radiation coming from the patient interacts with the camera, elements of the camera scintillate or give off light. This light is converted to an electric signal and we get a picture on a computer monitor. Gamma (the root of Gamma Camera): Gamma refers to Gamma rays. Gamma rays are the type of radiation given off by the radiopharmaceutical. The gamma rays cause the elements of the gamma camera to scintillate. A radioactive compound called Technetium (99m-TC) is the radioactive element used in most scintigraphic applications. The technetium is bound to other agents causing it to localize in different areas of the body. The combination of the technetium and the other agent is called a radiopharmaceutical. For example technetium can be combined with a diphosphonate (99mTC-MDP) a phosphate that localizes in bone. Different radiopharmaceuticals are used for different tests (more to come on this mater.) In general, the radiopharmaceutical localizes in a particular organ of interest depending on the examination. If there is increased accumulation of the radiopharmaceutical in an organ, this is an indication that there is increased activity of that organ. That said, nuclear medicine is used to evaluate function. All of the other modalities (pretty much) only give us anatomic information.

5 Best Use: There are literally dozens of uses for nuclear scintigraphy. The advent of CT, MRI, and ultrasound have rendered many of them obsolete. However, some applications see almost daily use at Auburn. Two of these are: Thyroid Scintigraphy: Technetium in the form of pertechnetate (99m-TCO4) localizes in the thyroid gland. Increased activity is associated with an overactive thyroid. This is seen with hyperthyroidism or thyroid tumors. Bone Scintigraphy: Technetium is combined with a diphosphonate (99mTC- MDP) which localizes in bone. Increased activity is associated with inflammation, tumors, or trauma. Keep in mind that bone scintigraphy is much more sensitive than radiographs but not as specific. What that means is that a hot spot, is a hot spot, is a hot spot. Tumors, inflammation, and trauma can all look exactly the same on a bone scan. Disadvantages: It doesn t take a genius to realize that having a radioactive pet will scare an owner, and it should. I said it before and I ll say it again the least amount of radiation you are exposed to the better. Animals having a scintigraphic examination must be isolated for 48 hours or more. Also, anesthesia is sometimes required to keep them still while we acquire the images. The images that are produced are small and the resolution is crummy. (This is all right though because we are looking for functional information. If a high-resolution image with anatomic detail is needed, use a different test. ) Last, the equipment costs are pretty high and some degree of manpower is necessary to ensure quality control. Again, most of you will not have this one at your practice. Terminology: We describe nuclear medicine images in terms of activity or accumulation. An area of increased activity or localization creates a hot spot on the image. Areas of decreased activity create a cold spot.

6 Computed Tomography Also known as CT or CAT scan, this modality is becoming more and more common in veterinary medicine. Many big cities (and some small ones) have referral institutions with a CT scanner. If you want to buy one you can find one on e-bay but you have to be licensed by the state to use it! Basis: X-rays (just like in diagnostic radiography) are passed through a patient. A computer measures the attenuation of the x-ray beam as it passes through the patent while the x-ray tube travels around the patient. The images consists of multiple slices that are free of the problems of the superimposition of structures we had with plain ol radiographs. Additionally, there is much better differentiation between tissues of a similar radiographic density; yet another problem with conventional radiographs that is solved. Since the images are digital, they an be reformatted in a number of ways. 3D reconstructions will wow anyone. Best Use: Well just about any part of the body can be evaluated with CT. Abdomen, lungs, brain, joints anything. In human medicine CT is often first line diagnostic imaging for things like appendicitis, acute head trauma, lung tumors, and pancreatic disease. However, we are not so lucky. Since we have to anesthetize out patients (so they wont move during the process), the use of CT is more limited. You don t want to anesthetize a sick patient (because they might die) and you don t want to anesthetize a healthy one if you can avoid it. Oh Well! Our most common uses are: Nasal Cavity Skull tympanic bulla Tumor staging Some musculoskeletal conditions such as fragmented medial coronoid process Disadvantages: Again, the big disadvantage is that the animal must be anesthetized. Another is cost and availability of a CT scanner. We charge xxx plus the cost of anesthesia for a CT of the nasal cavity. Last, the exposure to radiation is considerable (nearly 20 times that of a radiograph). Terminology: We describe CT images in terms of opacity.

7 Magnetic Resonance Imaging Also known as MRI or MR, this is the newest, most advanced of the imaging modalities. Fortunately, we have an MRI unit at Auburn. Many vet schools are not so lucky. I can say with confidence, however, that none of you will own an MRI in the near future. The costs associated with these machines are staggering. Nonetheless, the technology is prevalent in human medicine and, undoubtedly, your clients will ask you about MRI. A working knowledge of the basics is essential for the renaissance veterinarian. Basis: The patient is placed in a large magnet and radiofrequency pulses are administered to the patient. This process serves to align the magnetic poles of the hydrogen molecules in the body and then knock them down like a weeble-wobble. As the protons regain alignment with the magnetic field they give off energy (resonate). This energy is used to form an image. If that doesn t make sense, don t worry about it the previous explanation was probably too simplified to be of any use anyway. We would need a full lecture to even glean the surface of how an MRI works. What you do need to know is that MRI gives us the best contrast resolution of soft tissues of any of the modalities we have discussed. It can also be used to image flowing blood. MRI does a poor job imaging bone since the hydrogen molecules in bone do not resonate so they do not produce a signal. But MRI is used in orthopedics to assess ligaments and other soft tissue structures of the joint. Best Use: MRI can be used to image just about any soft tissue in the body. It is established as the superior imaging modality of joints, spinal cords, and brains. At the AUCVM, we have a low field MRI (0.067Tesla) which limits the resolution of images and makes acquisition times long. The principle use for MRI at AUCVM is for brain imaging. Terminology: We describe MRI in terms of signal intensity.

Radiologic Imaging Magnetic Resonance Imaging (MRI)

Radiologic Imaging Magnetic Resonance Imaging (MRI) Radiologic Imaging X-ray has always been the golden rule in diagnosing and treating podiatric patients. Unfortunately, for some patients the diagnosis is not as evident. That is when we need to utilize